1. Field of the Invention
The present invention relates to a radiation imaging apparatus suitable for a variety of applications, such as medical diagnosis and industrial non-destructive inspection, and to a control method for the radiation imaging apparatus. In the present invention, radiation includes electromagnetic radiation, such as X-ray and γ-ray, α-ray, and β-ray.
2. Description of the Related Art
Generally, X-ray radiography systems used in hospitals are of a film radiography type and an image processing type. In a film radiography type system, a patient is irradiated with X-rays and a film is exposed to the X-rays transmitted through the patient. In an image processing type system, the X-rays transmitted through the patient are converted into electrical signals for digital image processing.
One X-ray radiography system of the image processing type is a radiation imaging apparatus including a phosphor for transforming X-rays into visible light and a photoelectric conversion device for converting the visible light into electrical signals. The phosphor is exposed to X-rays transmitted through a patient to transform body information of the patient into visible light, and the transformed body information of the patient is output as electrical signals from the photoelectric conversion device. The electrical signals can further be converted into digital signals by an analog-to-digital (A/D) converter so that X-ray image information used for in recording, displaying, printing, diagnosis, etc., can be processed as digital data.
Recently, radiation imaging apparatuses including an amorphous silicon semiconductor thin-film photoelectric conversion device have been commercially available, as disclosed in Japanese Patent Laid-Open No. 9-307698.
FIG. 8 is a two-dimensional circuit diagram of a photoelectric conversion substrate of the related art including photoelectric conversion elements and switching elements made of amorphous silicon semiconductor thin films. For simplification of illustration, 3×3 pixels, i.e., nine pixels, are shown in FIG. 8.
In FIG. 8, a photoelectric conversion circuit unit (or a radiation detection circuit unit) 701 includes photoelectric conversion elements S1-1 to S3-3, switching elements (or thin-film transistors (TFTs)) T1-1 to T3-3, gate lines G1 to G3 for turning on or off the switching elements (TFTs) T1-1 to T3-3, signal lines M1 to M3, and a Vs line for applying an accumulation bias to the photoelectric conversion elements S1-1 to S3-3. The Vs line is biased by a power supply Vs. A shift register SR1 applies driving pulse voltages to the gate lines G1 to G3. A voltage Vg(on) for turning on the switching elements (TFTs) T1-1 to T3-3 and a voltage Vg(off) for turning off the switching elements (TFTs) T1-1 to T3-3 are supplied to the shift register SR1 from the outside.
A reading circuit unit 702 amplifies and serial converts parallel signals output from the signal lines M1 to M3 in the photoelectric conversion circuit unit 701, and outputs the resulting signals. The reading circuit unit 702 includes switches RES1 to RES3 for resetting the signal lines M1 to M3, amplifiers A1 to A3 for amplifying the signals on the signal lines M1 to M3, sample-hold capacitors CL1 to CL3 for temporarily storing the signals amplified by the amplifiers A1 to A3, switches Sn1 to Sn3 for performing sample hold, buffer amplifiers B1 to B3, switches Sr1 to Sr3 for converting parallel signals into serial signals, a shift register SR2 for supplying pulses for serial conversion to the switches Sr1 to Sr3, and a buffer amplifier Ab for outputting the converted serial signals.
FIG. 9 is a timing chart showing the operation of the photoelectric conversion device shown in FIG. 8.
In a photoelectric conversion period, all photoelectric conversion elements S1-1 to S3-3 are in a biased state to the electric potential of the power supply Vs and are irradiated with X-rays. The photoelectric conversion elements S1-1 to S3-3 generate charges (electrons and holes) proportional to the X-ray dose. At this time, the switching elements (TFTs) T1-1 to T3-3 are still in the off position, and the generated charges are accumulated in inter-electrode capacitors of the photoelectric conversion elements S1-1 to S3-3.
In a read period, a reading operation is performed in the order of the photoelectric conversion elements S1-1 to S1-3 in the first row, the photoelectric conversion elements S2-1 to S2-3 in the second row, and the photoelectric conversion elements S3-1 to S3-3 in the third row. First, the shift register SR1 supplies gate pulses to the gate line G1 associated with the switching elements (TFTs) T1-1 to T1-3 in the first row to perform the reading operation of the photoelectric conversion elements S1-1 to S1-3. The high level of the gate pulse corresponds to the voltage Vg(on) supplied from the outside. The switching elements (TFTs) T1-1 to T1-3 are then turned on, and the signal charges accumulated in the photoelectric conversion elements S1-1 to S1-3 are transferred to the signal lines M1 to M3. The signal charges transferred to the signal lines M1 to M3 are amplified by the amplifiers A1 to A3. Although not shown in FIG. 8, reading capacitors are added to the signal lines M1 to M3, and the signal charges are transferred to the reading capacitors via the switching elements (TFTs) T1-1 to T1-3. For example, the reading capacitor added to the signal line M1 corresponds to the sum of inter-electrode capacitors (Cgs) between gates and sources of the switching elements (TFTs) T1-1 to T3-1 connected to the signal line M, i.e., three capacitors.
An SMPL signal is turned on to transfer the signal charges to the sample-hold capacitors CL1 to CL3, and the SMPL signal is off to hold them. When pulses are applied from the shift register SR2 to the switches Sr1, Sr2, and Sr3 in this order, the signals held in the sample-hold capacitors CL1 to CL3 are output as an output signal Vout from the amplifier Ab in the order of the sample-hold capacitors CL1, CL2, and CL3 to an A/D conversion circuit unit. Thus, the photoelectric converted signals of the photoelectric conversion elements S1-1, S1-2, and S1-3 in the first row are sequentially output.
After the signal lines M1 to M3 are reset to a ground (GND) potential, the shift register SR1 supplies gate pulses to the gate line G2 associated with the switching elements (TFTs) T2-1 to T2-3 in the second row to perform the reading operation of the photoelectric conversion elements S1-1 to S1-3. The reading operation of the photoelectric conversion elements S2-1 to S2-3 in the second row and the reading operation of the photoelectric conversion elements S3-1 to S3-3 in the third row are sequentially performed. The sample hold of the signals on the signal lines M1 to M3 into the sample-hold capacitors CL1 to CL3 allows the signal charges of the photoelectric conversion elements S2-1 to S2-3 in the second row and the photoelectric conversion elements S3-1 to S3-3 in the third row to be transferred using the shift register SR1 while serially converting the signals for the first and second rows using the shift register SR2. Specifically, the signals on the signal lines M1 to M3 are sampled and held in the sample-hold capacitors CL1 to CL3 by the SMPL signal for the first row, and the signal lines M1 to M3 are reset to the GND potential by a cres signal (RC1). Then, gate pulses are applied to the gate line G2. Thus, while the shift register SR2 performs serial conversion on the signals for the first row, the shift register SR1 transfers the signal charges of the photoelectric conversion elements S2-1 to S2-3 in the second row.
Therefore, the signal charges of all photoelectric conversion elements from the first row to the third row can be output to obtain a single still image. The photoelectric conversion period and the read period, in which an image is obtained, are referred to a reading operation period.
In an X-ray imaging apparatus of the related art, generally, photoelectric conversion elements made of amorphous semiconductor as a main material, such as amorphous silicon or amorphous selenium, do not provide a stable photoelectric conversion in proportion to the X-ray dose due to dark current variations immediately after applying a bias.
Therefore, although not shown in FIG. 9, the X-ray imaging apparatus of the related art has an idling operation period of at least several seconds prior to the photoelectric conversion period. The idling operation period alternately includes a wait period and a read period. In the wait period, X-rays are not emitted or charges are not read. The idling operation period provides a stable dark characteristic in the photoelectric conversion period during which X-rays are emitted.
In the X-ray imaging apparatus, a radiographer, such as an X-ray engineer, presses an X-ray emission switch (or irradiation switch) to emit X-rays. However, the idling operation period makes it difficult to synchronize a reading operation of the X-ray imaging apparatus with X-ray emission. For example, if the irradiation switch is pressed in the read period of the idling operation period, X-rays enter the photoelectric conversion circuit unit 701 while reading signal charges.
In a control method for avoiding this problem, when a radiographer, such as an X-ray engineer, presses the irradiation switch, a reading operation ends in the idling operation period, and, after the confirmation, the idling operation period transitions to the photoelectric conversion period in which X-rays are emitted.
However, this control method causes a delay between pressing the irradiation switch and emitting X-rays. A slight delay may be negligible. However, medical X-ray imaging apparatuses for use in simple thoracic radiography generally have a wide radiographic range and a large number of pixels, and therefore require approximately 0.1 to 1 seconds for reading. X-ray radiography using such medical X-ray imaging apparatuses can make radiographers uncomfortable. Another problem is that a desired image cannot be obtained due to a delay although a radiographer presses the irradiation switch at a desired time. Therefore, the chance of taking the best radiographs may be lost.
Moreover, it is necessary to electrically connect the X-ray imaging apparatus and an X-ray generator via a connection cable or the like to synchronize X-ray emission with a reading operation of the X-ray imaging apparatus after the irradiation switch is pressed by a radiographer. The connection cable is inconvenient for the radiographer particularly in radiography using a portable X-ray imaging apparatus, such as a film cassette type apparatus, and reduces the radiographic efficiency. An X-ray imaging apparatus having a connection cable can break down due to the cable being tripped over or walked on, and the radiographic operation is interrupted until the broken apparatus is repaired.